1. Field of the Invention
The present invention relates generally to semiconductor integrated circuits, and more specifically to a method for forming field effect devices in a semiconductor substrate.
2. Description of the Prior Art
To increase the number of devices which can be fabricated on a single integrated circuit chip, lower the cost of integrated circuit devices, and improve performance, transistors continue to be made smaller. For feature sizes less than approximately one micron, especially for sizes of approximately one-half micron and below, a number of performance problems are encountered due to the physics of semiconductors and small transistor sizes.
As known in the art, short channel effects degrade transistor performance. In order to counter these degradations, it is known in the art to form lightly doped drain (LDD) regions between the transistor channel and the more heavily doped source/drain regions. This provides a more lightly doped, graded, impurity region near the edges of the gate, decreasing the impact of problems such as the hot electron effect.
With device feature sizes greater than approximately one micron, later thermal processing cycles cause some diffusion of the implanted lightly doped drain region. This diffusion provides an impurity gradient which helps minimize the effect of high electric fields and enhances device performance.
With smaller geometries, such as one-half micron and below, later thermal cycles are minimized. The thermal budget allowed for very small feature size devices must be decreased because the source/drain junctions must be shallower than is the case with larger devices. A lower thermal budget means that the lightly doped drain regions diffuse to a lesser degree than is the case with larger devices, and this lesser diffusion does not provide the same LDD gradient profile as occurs with larger devices. Thus, with smaller devices, a sharper gradient occurs near the channel, which means that higher electric fields still occur in this region. The shorter channel used with smaller geometry devices also tends to increase the electric field gradient, further exacerbating the problem.
It is known in the art to provide lightly doped drain regions having multiple steps of impurity levels. For example, U.S. Pat. No. 4,356,623, issued Nov. 2, 1982 to W. R. Hunter, describes a technique for fabricating field effect devices which have stepped lightly doped drain regions. These are formed by forming, in succession, multiple sidewall regions on the gate electrodes, followed by impurity implants after each layer of sidewall formation. Formation of multiple sidewalls in this manner, and the attendant damage to the gate and to underlying structures caused by multiple anisotropic etch back steps, can have an adverse impact on device functionality.
It would be desirable to provide a method for fabricating very small transistors which provides graded impurity density in the lightly doped drain regions. It would be further desirable for such method to be simple to perform and to provide a highly functional device.